Unlike most phablets, you can easily use the iPhone with one hand. However, some gestures such as pinch to zoom require the use of two fingers. By turning on Assistive Touch, you can perform these gestures--and create your own--with just one finger.
Open Settings>General>Accessibility and turn on AssistiveTouch. A small floating black box with a white circle will appear. Press this, and a larger box appears with shortcuts for Siri, Device, Home, and Favorites. Select Favorites, then Pinch, to enable one-finger pinch-to-zoom. A blue bar with circles at either end will appear on-screen; you can then use your thumb to pinch-zoom and rotate images.